feat(platformer): add stomp-or-die enemy collisions, live HUD, GameOver state

The previous platformer example drew enemies but had almost no
interaction with them: only enemy 1 had a stomp check, the stomp
window was unreachable under the default +1-px-per-frame-plus-a-
jump-every-40-frames autopilot, contact from any other angle was
a silent no-op, and the header comment promised a "title → playing
→ game-over state machine" that didn't actually exist. The README
demo gif and the committed golden both froze that state — a level
the player could walk through indefinitely with no consequence.

Flesh the enemy interaction model out into something real:

- `resolve_enemy_hit(e_sx)`: one helper, called symmetrically for
  both enemies. Computes the player/enemy hitbox overlap (horizontal
  in `e_sx ∈ (72, 96)`, vertical in `player_y ∈ (152, 176)`) and
  branches three ways — falling onto the head is a stomp bounce
  (`rise_count = 6`, `fall_vy = 0`, `stomp_count += 1`, `play Boing`);
  overlap while `rise_count > 0` is a grace pass-through so the
  stomp bounce itself can't retrigger contact on the same enemy;
  anything else (walking into the side, standing on the ground
  against the enemy) is fatal — `alive = 0` and `play hit`.

- New `GameOver` state: draws four enemy tiles across the middle
  of the screen plus a coin row sized to `stomp_count`, stops the
  music, lingers 60 frames then auto-retries, and also honours
  Start for an instant retry.

- Proximity-based autopilot: pre-jump when an enemy is exactly 19 px
  ahead (`e1_sx == 99` or `e2_sx == 99`), capped at two jumps per
  life by `auto_jumps < AUTOPILOT_JUMPS`. Tuning: a JUMP_RISE=12,
  GRAVITY_CAP=4 jump lands the player's feet at enemy-head height
  exactly 21 frames after lift-off, by which point the autopilot
  camera has scrolled the enemy under the player. The first jump
  fires on Playing frame 1 and stomps enemy 1 on frame 22; the
  second fires on Playing frame 101 and stomps enemy 2 on frame
  122. After that the autopilot is exhausted and the third enemy
  encounter (camera wraps back past enemy 1) is fatal — the
  golden harness now sees the full stomp, stomp, die, retry, stomp
  loop instead of a frozen walk.

- Live HUD: up to four coin sprites in the top-left, one per
  stomp, rendered both during `Playing` and on the `GameOver`
  screen so the score is visible in the death frame. `Playing`'s
  player draw is now guarded by `if alive == 1` so the hero
  disappears on the fatal-contact frame and the enemy that killed
  them is visible underneath.

Verified with a per-frame ZP trace through the patched puppeteer
+ jsnes harness: first stomp at emu frame 44 (camera_x=22), second
at emu frame 144 (camera_x=122), death at emu frame 283 (camera_x=5
after a 256-px wrap), `Playing` restart at emu frame 343, third
stomp at emu frame 365. All 22 emulator goldens still match after
the update, and `docs/platformer.gif` regenerated from the new ROM
now shows two clean stomps, a clean side-collision death, the
GameOver screen, and the retry cycle all inside the 6-second demo
window.

Golden updates:
- `tests/emulator/goldens/platformer.png` — the frame-180 capture
  now shows the hero walking forward with a two-coin HUD after
  both autopilot stomps (previously: a frozen bouncing hero).
- `tests/emulator/goldens/platformer.audio.hash` — the track now
  includes two `Boing` stomp bounces, which shifts the hash.
- `examples/platformer.nes` — rebuilt from the rewritten source.

Also updates the platformer rows in `README.md` and
`examples/README.md` to match the new gameplay.

https://claude.ai/code/session_013Bi4H4YQ5or5HtMB4doUFi

examples/README.md

@@ -28,7 +28,7 @@ Open any `.nes` file in an NES emulator ([Mesen](https://www.mesen.ca/), [FCEUX]
 | `mmc1_banked.ne` | MMC1, banks, multiply | Banked mapper with software multiply |
 | `palette_and_background.ne` | palette, background, set_palette, load_background | Reset-time initial load plus vblank-safe runtime swaps |
 | `friendly_assets.ne` | named colours, grouped palette, pixel art, tilemap+legend, palette_map, scalar sfx pitch, note-name music | Exercises every "friendlier" asset syntax at once — the `palette` uses `bg0..sp3` + a shared `universal:`, the sprite is authored as ASCII pixel art, the background uses a `legend { ... } + map:` tilemap with a `palette_map:` for attributes, the sfx uses a scalar `pitch:` + `envelope:` alias, and the music uses note names (`C4, E4 40, rest 10`) with a `tempo:` default. |
-| `platformer.ne` | **every subsystem** | End-to-end side-scrolling demo: custom CHR tileset, full 32×30 nametable with per-region attribute palettes, 2×2 metasprite hero with gravity/jump physics, wrap-around horizontal scrolling, enemies, coin pickups, user-declared SFX + music, and a Title → Playing state machine with an autopilot so the headless harness still hits interesting gameplay at frame 180. Regenerate the tile art with `cargo run --bin gen_platformer_tiles`. |
+| `platformer.ne` | **every subsystem** | End-to-end side-scrolling demo: custom CHR tileset, full 32×30 nametable with per-region attribute palettes, 2×2 metasprite hero with gravity/jump physics, wrap-around horizontal scrolling, stomp-or-die enemy collisions with a live stomp-count HUD, coin pickups, user-declared SFX + music, and a Title → Playing → GameOver state machine with a proximity-based autopilot so the headless harness cycles through stomp, stomp, die, and retry inside six seconds. Regenerate the tile art with `cargo run --bin gen_platformer_tiles`. |
 
 ## Emulator Controls